Radio frequency (RF) switches are important building blocks in many wired and wireless communication systems. RF switches are found in many different communication devices such as cellular telephones, wireless pagers, wireless infrastructure equipment, satellite communications equipment, and cable television equipment. As is well known, the performance of RF switches may be characterized by one of any number operating performance parameters including insertion loss and switch isolation. Performance parameters are often tightly coupled, and any one parameter can be emphasized in the design of RF switch components at the expense of others. Other characteristics that are important in RF switch design include ease and degree (or level) of integration of the RF switch, complexity, yield, return loss and, of course, cost of manufacture.
FIG. 5 shows a pseudomorphic High Electron Mobility Transistor (pHEMT) RF switch 500 according to the prior art. Switch 500 includes an RF common (RFC) input node 501 and two RF output nodes RF1 (502), RF2 (503). Coupling/DC blocking capacitors are also shown at each of the nodes 501-503 but are ignored for purposes of the instant description. Those skilled in the art will appreciate that such capacitors impede the passage of DC current, yet do not appreciably impact an AC signal.
As further shown, several transistors M51, M52, M53, and M54 are arranged to effect RF communication between RFC 501 and RF1 502, or between RFC 501 and RF2 503. Specifically, M51 is arranged between RFC 501 and RF1 502, M52 is arranged between RF1 502 and ground, M53 is arranged between RFC 501 and RF2 503, and M54 is arranged between RF2 503 and ground. Each of the transistors M51-M54 includes by-pass resistors (which are not labeled with reference numerals) connected between respective drain and source terminals.
Two control voltages VC1 and VC2 applied, respectively, to the gates of M51 and M53 control which path (RFC to RF1 or RFC to RF2) will be taken by an RF AC signal input at RFC 501. In the configuration shown, VC1 is 3.3V, which turns M51 on. VC2 is 0V, which turns M53 off. In this configuration the RF path is configured to be RFC to RF1. M52 and M54 operate to enable either an isolation branch or a shunt branch depending on which path (RFC to RF1 or RFC to RF2) is selected. That is, when VC1 is high (3.3V), VC1B (the control voltage applied to the gate of M52) is controlled to be low, e.g., 0V. With VC1B low, M52 is off thereby isolating the path between RFC 501 and RF1 502. Meanwhile, VC2B is set high thus turning M54 on and creating an AC signal shunt path between output node RF2 503 and ground to ensure that no signal (or very little) is present at RF2 503, when RF2 503 is not selected to output the AC signal received at RFC 501. The several control voltages VC1, and VC2 are applied via respective resistors (which are also not labeled with reference numerals).
For high power operation for the switch shown in FIG. 5, the voltage of node 525 must be high enough to not only make control switch M51 forward biased with a positive Vgs over drive (VC1—“the voltage at node 525) to reduce turn-on insertion loss, but also to maintain sufficient reverse bias of control switch M53 (0—“voltage at node 525”) to avoid turn-on during high power voltage swings.
For a depletion pHEMT device, threshold voltage (Vth) is about −1V. Due to the relatively large leakage current in a pHEMT device, two back-to-back diodes 520, 521 form a Kirchoff Voltage Law (KVL) node. Specifically, with VC1=3.3 voltage volts, the drop across diode 520 is approximately 0.7V, which causes a voltage of 2.6V to be present at node 525. With a voltage of 2.6V at node 525, Vgs for M52 (which is reversed biased) is 0V-2.6V, or −2.6V. The point here is that as a result of the leakage current, node 525 is set at 2.6V which is suitable for handling high power RF switch operations, and as a result, no auxiliary biasing is needed to support a high power RF switch implemented with pHEMT devices.
Unlike pHEMT device-based RF switches, silicon-based RF switches permit much less leakage current. As such, silicon-based RF switches operating in high power scenarios require special biasing circuitry and voltages to operate properly. There is accordingly a need to provide cost effective ways of providing such biasing circuitry and voltages in silicon-based RF switching devices.